CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Making fluid slightly compressible

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 18, 2022, 22:02
Default Making fluid slightly compressible
  #1
New Member
 
Rick
Join Date: Aug 2022
Posts: 18
Rep Power: 3
Rick is on a distinguished road
Hi Everyone,

I am trying to simulate an incompressible fluid flow in a pipe. The problematic part of this simulation is that the outlet gets closed at some point during this transient simulation, and we have one inlet and no outlet. During this time, the inlet flow is supposed to inflate the walls by increasing the pressure inside the pipe. I tried to simulate this using an incompressible fluid but got convergence issues. After reading related posts here, I realized that making the fluid slightly compressible improves convergence. I did this using the following expression for the fluid density:

density=(Absolute pressure)/(R*Temperature)

Since the temperature is equal to 310K and absolute pressure should be near atmospheric pressure, to get the fluid density of 1050 kg/m3, the value of R needs to be equal to 0.3113.

Would you please let me know if this way of making the fluid slightly compressible causes any stability problems? I am asking this because my simulation can only be converged if the density and pressure URFs is less than 0.3.

Many thanks in advance!
Rick is offline   Reply With Quote

Old   September 19, 2022, 00:53
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Before you make weird changes to the fluid model - are you sure your model is well-posed? If your model is not well posed then you are never going to get useful results from it, even if it does pretend to converge. To answer this question, can you post your CCL or output file so we know what you are doing?

If you make the fluid mildly compressible be aware that you will then get compressible fluid pressure waves bouncing around everywhere, and your time step will be very small so you can resolve these waves. The speed of these pressure waves will be the acoustic velocity and you will need a CFL number of around 1 to resolve them - so this will tell you the time step size you will need.

Also - why make up a compressibility function when a real one already exists? I would recommend you use the bulk modulus for water for your compressibility function, as this is physically real. It is the cause of things like water hammer so it is very well known (https://en.wikipedia.org/wiki/Water_hammer).
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Old   September 19, 2022, 22:05
Default
  #3
New Member
 
Rick
Join Date: Aug 2022
Posts: 18
Rep Power: 3
Rick is on a distinguished road
Dear Glenn,

Thank you very much for your attention.

I am trying to perform this simulation using fluent. I asked this question here because I thought it is a general question that could be helpful for CFX users as well. In fluent there are just two options to model compressible fluids, the ideal gas model and the compressible-liquid option.

Since this is a Multiphysics simulation, I don't know where I should search for Fluent's output file. This was so straightforward in CFX. I'll keep you posted.

Thanks again for your help.
Rick is offline   Reply With Quote

Old   September 19, 2022, 22:10
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
If you are using Fluent then we will not be able to help you with the details of the simulation. Try the Fluent forum for that.

But the second and third paragraphs of my previous post are applicable to any CFD code, so those comments should be relevant.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[swak4Foam] swak4foam for OpenFOAM 4.0 mnikku OpenFOAM Community Contributions 80 May 17, 2022 09:06
Question about adaptive timestepping Guille1811 CFX 25 November 12, 2017 18:38
Error in Two phase (condensation) modeling adilsyyed CFX 15 June 24, 2015 20:42
Overflow Error in Multiphase Modelling with Two Continuous Fluids ashtonJ CFX 6 August 11, 2014 15:32
OpenFOAM on MinGW crosscompiler hosted on Linux allenzhao OpenFOAM Installation 127 January 30, 2009 20:08


All times are GMT -4. The time now is 07:02.